Non-gary scale anti-aliasing method for laser printers
Contents
1. on pixel prior to the transition point The duration of this pulse of laser energy is insufficient to charge this pixel location enough to attract toner however in the over lap region sufficient charge may be added to the charge already deposited by the lower raster linen to alter the boundary of the region in which toner sticks to the drum Thus in effect the location of a horizontal edge of the image may be controlled to within less than the width of a full pixel The most significant advantage of using the smooth ing method of the present invention in conjunction with a laser printer having unequal resolutions in the hori zontal and vertical dimensions is that quality of the printed image is nearly equivalent to that of an image printed on a much higher resolution printer having equal resolutions in the horizontal and vertical dimen sion Although the resolution of the laser printer is 600 x 300 dpi the image produced using the method of the present invention will more closely resemble 600 600 dpi print quality An additional advantage is that this increase in quality is accomplished using the modified pixel representation in a frame buffer memory that occupies half of the space of the frame buffer mem ory required to produce an equivalent pixel representa tion for a higher resolution printer As a result less memory is needed for the printer controller card and the printer controller card is less expensive In the preferred emb2. a variety of personal com puters such as an IBM PC XT AT 386 or PS 2 avail able from International Business Machines Armonk N Y or an Apple Macintosh available from Apple Computers Inc Cupertino Calif The personal com puter 12 may be provided with a desktop publishing software package such as Ventura Publisher available from Xerox Corporation PageMaker available from Aldus Corporation or GEM Desktop Publisher avail able from Digital Research Inc The laser printer 14 may be any type of Canon CX or SX laser printer such as the HP LaserJet Series II available from Hewlett 5 041 848 5 Packard the LaserWriter available from Apple Com puter Inc or the Canon LBP8 III available from Canon In normal operation without a printer controller card the personal computer 12 includes a main PC mother board 16 and a serial port 18 through which the personal computer 12 communicates with the internal controller 20 of the laser printer 14 The computer 12 sends a serial bit stream to the laser printer 14 over an RS 232 C serial interface 22 in the form of various com mands and data that represent the image to be printed by the laser printer 14 The internal controller 20 de codes the serial bit stream and generates the control signals that will be supplied to the laser 24 to drive the print engine 26 that creates the printed images corre sponding to the various commands and data received by the internal controller For a mo
3. 69 FILL is again greater than or equal to 1 and FILL 25 20 1 20 7 20 8 20 9 20 Although the preferred embodiment of the present invention is described with respect to the filling or rast erizing of an ideal outline it should be noted that the method of the present invention may be applied equally as well as to any filled polygon where it is desired to smooth the horizontal components of the polygon when 20 25 30 35 45 50 55 pixels in from ether end of the line segment 60 All of 60 the pixels to the left of transition point 56 are off and all of the pixels to the right of transition point 56 are In contrast in FIG 6c the pixels 52 along the line segment 60 are selectively turned off and on around the transition point 56 For purposes of estimating the area of each pixel 52 to be turned on an assumption is made that the increase in area for each pixel is represented by the slope of the line 1 20 Hence for pixel 61 the area 65 printing the image out on a printer having unequal hori zontal and vertical resolutions In this situation the ideal outline could be assumed to be a series of lines segments constructed around the outermost edges of the horizontal components of the filled polygon Although the description of the preferred embodi ment has been presented it is contemplated that various changes could be made without deviating from the spirit of the present invention Accor
4. As the decision is made whether to turn a pixel 52 on or off a fill value representing the area between edge of the ideal outline 54 and the edge of the pixel 52 is accumulated When the fill value is equal to or greater than the total area of a single pixel the pixel presently being operated on is turned on and the area of a single pixel is subtracted from the fill value In this embodiment the fill value acts like a running accumulator with the carry out of the accumulator being used to determine whether the pixel presently being processed should be turned on Any number of methods can be used to determine the amount of the fill value for each pixel For example instead of calculating the entire area inside the ideal 5 041 848 7 outline the height of the midpoint of the segment of the ideal outline in the pixel or the height of the intersection of the ideal outline with the right or left edge of the pixel could be used to estimate the value of the area inside the ideal outline Similar algorithms may be used to process short vector and curves in order to apply the smoothing method of the present invention to a variety of different shapes Referring now to FIGS 5 and 6 the operation of the preferred method of the present invention will be de scribed in greater detail At Start 100 an ideal outline 54 as shown in FIG 6a has been determined based upon the outline of the image or character to be printed It should be noted t
5. United States Patent Gilbert et al 11 Patent Number 5 041 848 45 Date of Patent Aug 20 1991 54 76 1 22 51 52 58 56 NON GARY SCALE ANTI ALIASING METHOD FOR LASER PRINTERS Inventors John M Gilbert 604 2nd St NE Minneapolis Minn 55413 Lawrence J Lukis 6366 150th St Leonard R Steidel 3473 Willow Beach Trail SW both of Prior Lake Minn 55372 Appl No 434 318 Filed Nov 13 1989 Int CES H04N 1 21 USE CL neg 346 108 346 1 1 358 296 Field of Search 346 108 107 R 76 L 346 160 1 1 358 296 300 302 298 355 202 References Cited U S PATENT DOCUMENTS 4 180 854 12 1979 Walden et al 364 200 4 682 189 7 1987 Purdy et al 364 900 BUFFER PROCESSOR Primary Examiner Mark J Reinhart 57 ABSTRACT A non gray scale anti aliasing method for smoothing the horizontal components of the edges of an image to be printed by a laser printer having unequal pixel reso lutions in the horizontal and vertical dimensions is ac complished by selectively modifying the on and off states of pixels on either side of each vertical transition point along the horizontal components of raster lines representing the edges of the pixel representation of the image A processor is provided with an ideal outline of the image and rasterizes the ideal outline of the image to determine which pixe
6. be printed by a laser printer having unequal resolu tions in the horizontal and vertical dimensions FIGS 2a 2b and 2c are schematic pixel representa tions of a shallow horizontal outline edge for two prior art pixel representations and the modified pixel repre sentation in accordance with the present invention FIGS and 3b are actual sample printouts of the same filled polygon having a shallow horizontal edge using a prior laser printing system and a laser printing system incorporation the present invention FIGS 4a and 4b are the same sample printouts as shown in FIGS and 3b shown in 4X scale FIG 5 is a flowchart showing the steps of the pre ferred embodiment of the smoothing method of the present invention FIGS 6a 6b and 6c are pixel representation of the outline of a filled polygon showing the ideal outline alone the ideal outline as filled using a prior laser print ing system and the ideal outline as filed using a laser printing system incorporating the present invention DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG 1 a typical desktop publishing system 10 having the printer controller card that exe cutes the software that comprises the preferred embodi ment of the present invention will be described The hardware for the desktop publishing system 10 is com prised of a personal computer 12 connected with a small inexpensive laser printer 14 The personal com puter 12 may be any one of
7. chniques incor porating a gray scale approach have been used in video displays to resolve this type of problem Unfortunately laser printers are binary imaging devices and are not 10 20 25 30 35 40 45 50 55 65 2 capable of implementing such gray scale techniques In larger and more expensive laser printers the problems associated with aliasing and non smooth edges may be resolved by using higher pixel resolutions in these print ers In the smaller inexpensive laser printers used with desktop publishing systems this approach is not used because of the associated increase in cost While the prior art printer controller cards have al lowed users of desktop publishing systems to signifi cantly increase the quality of their printed outputs by increasing the horizontal resolution of the existing laser printers used with such systems it would be advanta geous to be able to improve the quality of the outline edges of a character or image that occur in a near hori zontal orientation particularly if such an improvement could be accomplished without any additional hard ware that would increase the cost of such printer con troller cards SUMMARY OF THE INVENTION In accordance with the present invention a non gray scale anti aliasing method for smoothing the horizontal components of the edges of an image to be printed by a laser printer having unequal pixel resolutions in the horizontal and vertical dimensio
8. dingly it is in tended that the scope of the present invention be dic tated by the appended claims rather than by the descrip tion of the preferred embodiment We claim 1 A method for smoothing the edges of an image to be printed by a binary imaging printer having unequal pixel resolutions in the horizontal and vertical dimen sions comprising the steps of providing a processor means with an ideal outline of the image rasterizing the ideal outline of the image using the processor means to determine which pixels to turn on and which pixels to turn off in a frame buffer memory means that stores a pixel representation of the image to be printed by the printer smoothing the edges of the digital outline by selec tively modifying the on and off states of pixels on either side of each vertical transition point along the horizontal components of the edges of the pixel representation of the image to produce a modified pixel representation in the frame buffer means and communicating the modified pixel representation stored in the frame buffer memory means to the printer such that the printer may print the smoothed image 2 A method of smoothing the horizontal components of the edges of an ideal outline of an image to be printed by a binary imaging printer having unequal pixel resolu tions in the horizontal and vertical dimensions compris ing the steps of rasterizing the ideal outline of the image using a pro cessor means to proc
9. erein the step of commu nicating the modified pixel representation to the printer is accomplished by using a video interface to directly modulate the laser in the laser printer 7 The method of claim 1 wherein the step of smooth ing the edges of the digital outlines is accomplished by accumulating a fill value representing the area inside the ideal outline for the pixel currently being pro cessed 10 15 20 25 30 35 45 50 55 65 10 determining if the fill value is greater that the total area and if the fill value is greater than the total area turning on the pixel currently being processed and sub tracting the total area from the fill value 8 The method of claim 2 wherein the step of accumu lating a fill value representing the area inside the ideal outline for the pixel currently being processed is accom plished by calculating the entire area inside the ideal outline 9 The method of claim 2 wherein the step of accumu lating a fill value representing the area inside the ideal outline for the pixel currently being processed is accom plished by estimating the area inside the ideal outline based upon the height of the midpoint of a segment of the ideal outline that crosses the pixel currently being processed 10 The method of claim 2 wherein the step of accu mulating a fill value representing the area inside the ideal outline for the pixel currently being processed is accomplished by estima
10. ess a plurality of horizontal raster lines comprised of a plurality of pixels each pixel having a given total area to form a pixel representation of the image in a frame buffer the rasterizing being performed to smooth the horizon tal components of the ideal outline of the image by performing the following steps for each of the horizontal raster lines accumulating a fill value representing the area in side the ideal outline for the pixel currently being processed 5 041 848 9 determining if the fill value is greater than the total area and if the fill value is greater than the total area turning on the pixel currently being processed and sub tracting the total area from the fill value 3 The method of claim 1 wherein the processor means is comprised of a printer control card operably connected with a computer the printer control card including a co processor for rasterizing the ideal outline of the image the frame buffer memory means for stor ing the pixel representation of the image to be printed by the printer and a video interface for communicating the modified pixel representation stored in the frame buffer means to the printer 4 The method of claim 3 wherein the ideal outline is generated by the co processor in response to commands and data received from the computer 5 The method of claim 1 wherein the binary imaging printer is a laser printer including a laser and a marking engine 6 The method of claim 5 wh
11. hat in general the characters or im ages to be printed can be thought of as filled polygons or combinations of filled polygons It is also possible to use the method of the present invention with ideal out lines 54 whose edges are in the form of arcs and curves rather than straight lines At 102 the value SCAN LINE representing the number of horizontal scan lines 50 in the frame buffer 32 is set equal to zero At 104 the value PIXELCNT representing the number of pixels 52 in a scan line 50 and the value FILL representing the area of a single pixel 52 are also set to zero In this embodiment the area of a single pixel 52 is assumed to be one for simplicity of explanation It will be under stood that the value of FILL may be determined using any of the methods discussed above for estimating the area of the pixel 52 inside the ideal outline 54 At this point the software program that is imple menting the method of the present invention on the co processor 34 of the controller card 30 is ready to start rasterizing the ideal outline 54 to produce the hori zontal raster lines 50 that will be stored in the frame buffer 32 At 106 the value of the current pixel area that should be on because it is inside the ideal outlet 54 is added to FILL At 108 the value of FILL is compared to see if the current pixel should be turned on If so at 110 the current pixel is turned on and the value of the area of a single pixel 1 is subtracted fro
12. ler 30 to over ride the internal controller 20 of the laser printer and directly modulate the laser 24 It is this direct modula tion of the laser 24 that allows the printer controller 30 to increase the horizontal resolution of the laser printer 14 For a more detailed explanation of the operation the LX6 printer controller 30 reference is made to the LX User Guide which is incorporated herein by reference and is available from LaserMaster Corporation Referring now to FIGS 2a ec the differences be tween the smoothing method of the present invention and the prior art will be described Each of FIGS 2a 2c show a single raster horizontal raster line 50 comprised of a plurality of pixels 52 that may either be white or black Also shown is the ideal line 54 that forms the outline of the image or character to be printed In this case the ideal line 54 is the worst case version of a shallow horizontal line for the edge of an outline The prior art is represented by the 300 x 300 dpi raster line 50 generated by the internal controller 20 as shown in FIGS 2al and the 600 x 300 dpi raster line 50 generated by the LX6 controller 30 without using the method of the present invention It can easily be seen that there is a very definite and sharp vertical transition point 56 at the edges of the two pixels adjacent the point where the ideal line 54 crosses the midway vertical height of the pixels 52 This define and sharp ver
13. ls to turn on and which pixels to turn off in a frame buffer that stores a pixel representa tion of the iamge to be printed by the laser printer The modified pixel representation in the frame buffer is then communicated to the laser printer such that the laser printer may print the smoothed image 11 Claims 5 Drawing Sheets Sheet 1 of 5 5 041 848 Aug 20 1991 U S Patent 3JNION3 Id 39VJ luoss3ooud uaJ4na8 H3JNI 09 3WNVHJ H3TIOHINOO 9 X1 1HOd MHIS I aly U S Patent Aug 20 1991 Sheet 2 of 5 5 041 848 Q 52 1g 2b PRIOR ART Fig 2a PRIOR ART 52 52 U S Patent Aug 20 1991 Sheet 3 of 5 5 041 848 Fig PRIOR ART Fig 3b Fig 4a PRIOR ART 4x Fig 4b U S Patent Aug 20 1991 Sheet 4 of 5 5 041 848 START SET SCAN LINE O SET PIXELCNT 0 FILL O FILL FILL CURRENT PIXEL AREA THAT SHOULD BE ON TURN ON URRENT PIXEL FILL FILL 1 YES C INCREMENT PIXELCNT INCREMENT SCANLINE U S Patent Aug 20 1991 Sheet 5 of 5 5 041 848 Fig 6b PRIOR ART uis amm 60 5 041 848 1 NON GARY SCALE ANTI ALIASING METHOD FOR LASER PRINTERS TECHNICAL FIELD This invention pertains to methods of rasterization and fill techniques used to create images to be printed by laser printers More particularly this invention per tains to a
14. m FILL At 112 a check on PIXELCNT to see if processing is at the end of a raster line 54 If not then PIXELCNT is incremented and the loop is reentered at 106 If it is the end of a raster line then a check on SCANLINE is made at 116 to see if this is the last raster line in the frame buffer 32 If not the SCANLINE is incremented at 118 and the loop is reentered at 104 When the frame buffer 32 has been completely rasterized then control is returned from the software program via RETURN 120 To understand how the preferred embodiment of the present invention would work in practice and how it compares with the prior art reference is made o FIGS 6b and 6c It will be noted that the pixels 52 in these figures are rectangular indicating the non uniform hori zontal and vertical resolutions of the printer used to print these images Using the line segment 60 as an example it will be seen that the line segment 60 is one pixel high and 20 pixels long In this case the transition point 56 using the prior art rasterization methods is 10 8 inside the ideal outline 54 below the line segment 60 is 1 20th the area of an entire pixel 52 For pixel 62 the area is 2 20th etc By accumulating the values for each of the pixels in line segment 60 it can be seen that at pixel 66 the value of FILL 21 20 1 20 2 20 3 20 4 20 5 20 6 20 Conse quently pixel 66 is turned on and FILL now equals 1 20 The process is continued and at pixel
15. n the near vertical direction A primary objective of the present invention is to provide a non gray scale anti aliasing method for smpothing the horizontal edges of an image to be ywa 0 20 25 30 40 45 50 55 60 65 4 printed by a laser printer having unequal resolutions in the horizontal and vertical dimensions Another objective of the present invention is to in crease the quality of the image printed using a printer controller card without increasing the amount of frame buffer memory required to store the pixel representa tion of that image A further objective of the present invention is to increase the quality of the image printed using a printer controller card without requiring additional hardware circuitry on the printer controller card or the print engine of the laser printer Still another objective of the present invention is to implement the smoothing method of the present inven tion during the fill process of the rasterization of the pixel representation of the image without requiring any post rasterization processing or circuitry to accomplish the smoothing of the images These and other objectives of the present invention will become apparent with reference to the drawings the detailed description of the preferred embodiment and the appended claims DESCRIPTION OF THE DRAWINGS FIG 1 is a block diagram of laser printing system having a printer controller card for generating an image to
16. non gray scale anti aliasing method for smoothing the horizontal edges of an image to be printed by a laser printer having unequal resolutions in the horizontal and vertical dimensions BACKGROUND ART In recent years the use of smaller inexpensive laser printers with personal computers has created an entirely new industry referred to as desktop publishing Desktop publishing systems offer user the ability to format and print documents having complicated layouts using characters that have a variety of different fonts and type sizes While desktop publishing systems represent a significant advance in the art of publishing the standard resolution of the laser printers used with such systems typically a 300x 300 dpi Canon CX or SX based laser printer e g a HP LaserJet Series II printer was too poor to compete with traditional phototypesetting sys tems In an effort to improve the quality and speed of the smaller inexpensive laser printers used with desktop publishing systems a variety of printer controller cards have been introduced that may be installed either in the laser printer or in the personal computer Certain of the printer controller cards that are installed directly in the personal computer increase the quality and speed of the laser printers by using a separate co processor and page frame buffer to create a pixel representation of the image to be printed on the laser printer This image is then printed from the pixel repre
17. ns is accomplished by selectively modifying the on and off states of pixels on either side of each vertical transition point along the horizontal components of raster lines representing the edges of the pixel representation of the image The method of the present invention may be used by a pro cessor means that is provided with an ideal outline of the image and rasterizes the ideal outline of the image using the processor means to determine which pixels to turn on and which pixels to turn off in a frame buffer memory means that stores a pixel representation of the image to be printed by the laser printer The method of the present invention produces a modified pixel repre sentation in the frame buffer means that is communi cated to the laser printer such that the laser printer may print the smoothed image In essence the present invention smooths each of the vertical transition points between horizontal raster lines by adding and subtracting pixels around this transition point The pixel representation created in the frame buffer for each horizontal raster line is used to control the laser i the laser printer When the laser is turned on in response to an on pixel a generally circular laser beam image is reflected on the print drum of the print engine to transfer charge elements to the drum The charge elements attract and pick up the toner that is then transferred to the sheet of paper Although the pixel elements are thought of as squa
18. o processor 34 and a video interface 36 The co processor 34 receives the various commands and data represent ing the image to be printed from the PC mother board 16 via the bus 28 Because the bus 28 is a parallel bus the transfer rate is much faster than for the serial port 18 The co processor 34 decodes the commands and infor mation and produces a mathematically pure ideal out line corresponding to a filled polygon that wili repre sent each image or character to be printed The co processor 34 may also aid the personal computer 12 ina rendering lines graphics and characters as determined by the particular application software package cur rently being run on the personal computer 12 Once the ideal outline is generated it is then filled in or rasterized to produce a pixel representation of the image to be printed that is stored in the memory of the frame buffer 32 Any number of known fill techniques or algorithms may be used to fill in the pixels inside of the outline depending upon how overlapping images are to be handled When the laser printer 14 is ready to print a page or frame worth of images the pixel rep resentation in the frame buffer 34 is transferred to the laser printer 14 via the video interface 36 in the printer controller 30 that communicates directly with the laser 20 25 30 35 40 45 65 6 24 via a video cable 38 An internal switch 40 in the laser printer 14 allows the printer control
19. odiment of the present invention the smoothing of the horizontal components of the edges of the ideal outline is accomplished during the fill process as each horizontal raster line is being rasterized Typically the horizontal raster lines are processed from top to bottom in the frame buffer and the pixels are processed from left to right As each horizontal scan line of an ideal polygon outline is being rasterized the processor accumulates a fill value that represents the area of each pixel inside of the ideal outline that should be turned on or filled If the processor determines that the area of the fill value is greater than the area of a pixel the pixel presently being processed is turned on and the area of a pixel is subtracted from the fili value In this embodiment the fill value acts like a run ning accumulator with the carry out of the accumulator being used to determine whether the pixel presently being processed should be turned on Any number of methods can be used to determine the amount of the fill value for each pixel For example instead of calculating the entire area inside the ideal outline the height of the midpoint of the segment of the ideal outline in the pixel or the height of the intersection of the ideal outline with the right or left edge of the pixel could be used to esti mate the value of the area inside the ideal outline A similar method may be used to process edges and transi tions which occur i
20. re detailed explanation of the internal operation of the laser printer 14 refer ence is made to the LaserJet II Printer User s Manual Edition 1 available from Hewlett Packard Because of the inherent limitations on the speed at which the commands and data can be transferred over the serial interface and decoded by the internal control ler 20 the page throughput for the laser printer 14 is typically very low 6 pages per minute or less depend ing upon the formatting and type of information being printed To solve this problem and to simultaneously increase the quality and performance features of the laser printer various printer controller cards 30 have been introduced into desktop publishing systems such as the LX6 Professional available from LaserMaster Corporation Eden Prairie Minn The printer control ler card 30 of the preferred embodiment is located in the personal computer 12 and communicates directly with the PC mother board 16 via an internal bus 28 It will be recognized however that the printer controller board 30 may be located in the laser printer 14 or that the present invention may be implemented within the inter nal controller 20 of the laser printer 14 In the preferred embodiment the printer controller card 30 is an LX6 Professional printer controller card selected to match the particular type of laser printer 14 being used in the desktop publishing system 10 The LX6 controller consists of a frame buffer 32 a c
21. re or rectangular shapes in actuality the edges of the pixels typically bleed into one another to a significant degree This is due to the laser power function at the surface of the drum which typically decreases with the distance from the center of the focal point over a generally cir cular or elliptical area covering a space larger than the theoretical rectangular pixel Therefore the total laser energy impinging upon a given point on the drum may hav components contributed by more than a single sweep of the laser beam This effect will be especially pronounced in the region between the centerlines of two adjacent raster lines The total laser power imping ing on a given point determines the resulting amount of charge on the drum at that point which in turn deter mines whether toner will stick to that point or not 5 041 848 3 It is believed that the present invention takes advan tage of the physics of this process at the higher laser modulation rate generated by the printer controller cards to create the desired smoothing of the transition points For edges in the near horizontal raster line just below the outline edge some of those charge elements overlap onto the next horizontal raster line that contains the outline edge with the transition point In that next horizontal line the application of the laser in response to the modified pixel representation briefly turns the laser on for example for a selectively modified
22. sentation in the page frame buffer by directly controlling the modulation of the laser in the print engine of the laser printer An example of such a printer control card is the LX6 Pro fessional printer controller cards available from Laser Master Corporation Eden Prairie Minn the assignee of the present invention In some of the prior art printer controller cards such as the LX6 Professional it is possible to increase the horizontal resolution of the laser printer by controlling the modulation of the laser in the print engine of the laser printer By doing so the LX6 Professional printer controller cards can increase the horizontal component of the pixel resolution of the laser printer to 600 x 300 dpi and 1000 x 400 dpi depending upon the type of laser printer This increase in horizontal resolution signifi cantly improves the quality of the resulting printed image particularly for pixel transition points along the outline edges of a character or image that occur in the vertical or near vertical orientation Unfortunately the increased horizontal resolution does little to improve the quality of the outline edges of a character or image that are near horizontal in slope The noticeability and appearance of such vertical pixel transition points is typically referred to as aliasing that results in jagged or stairstep edges of the character or image outline oriented generally in the horizontal direction Traditionally anti aliasing te
23. tical transition point 56 creates jagged edges or steps that are visible to the human eye as shown for example in FIGS 3a and 4a In contrast the raster line 50 shown in FIG 2c that is generated by the LX6 controller 30 using the method of the present invention does not have such a sharp black and white vertical transition point 56 By selectively turning on pixels in the raster line 50 to the left of the transition point 56 and turning off pixels to the right of the transition point 56 the present invention smooths the transition point 56 as seen by the human eye as shown for example in FIG 3b and 48 Vertical and near vertical edges are smoothed in much the same fashion It will be understood that the smoothing of transitions may also be effective when operating at equal resolutions although the preferred embodiment of the present invention is most successful with unequal horizontal and vertical dimensions In the preferred embodiment the smoothing method of the present invention is applied by the co processor 34 as it is generating each horizontal raster line 50 to be stored in the frame buffer 32 Typically the horizontal raster lines 50 are processed from top to bottom in the frame buffer 32 and the pixels 52 are processed from left to right although it will be recognized that the raster lines 50 and pixels 52 may be processed in any direction so long as the processing is consistent throughout an entire page in the frame buffer 32
24. ting the area inside the ideal outline based upon the height of the left height of the left edge of a segment of the ideal outline that crosses the pixel currently being processed 11 The method of claim 2 wherein the step of accu mulating a fill value representing the area inside the ideal outline for the pixel currently being processed is accomplished by estimating the area inside the ideal outline based upon the height of the right edge of a segment of the ideal outline that crosses the pixel cur rently being processed
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